Ultrasound sensor for non-destructive control of metallurgical products

ABSTRACT

An installation for non-destructive control for a metal tube in which an ultrasound sensor includes transducer elements that can be excited each at selected times. A downstream circuit for processing the sensed signals analyzes a global response of the tube at ultrasonic excitation. The transducer elements are only excited to produce a single emission and the downstream circuit recovers the samples of the sensed signals each through a transducer element, to associate therewith successive times respectively offset, to calculate plural global responses of the tube at a single emission, by modifying the shifts between the successive times.

The invention relates to non-destructive control, especially inmetallurgy, and more particularly to the non-destructive control oftubes.

The manufacture of tubes is, as far as possible, made completelyautomatic. When manufacture is complete, the tubes are subjected to anon-destructive control by ultrasound waves, with the aim of selectivelydetecting one or more defects thereon, using the following tests:surface defects having a substantially longitudinal and/or transverseorientation, on the inside and/or on the outside; defects of thicknessand/or in the thickness; the inside and outside diameters are alsocontrolled.

In order to control the entire volume of the tubes, the tubes aresubjected to a helical relative movement with respect to the ultrasoundsensors and the ultrasound waves are emitted in bursts, at a fast rate,with a firing frequency called a “recurrence” frequency.

The indirect coupling of the sensor to the tube is effected in a liquid,in general water. In practice, in order to detect the various defectsmentioned above, sensors having longitudinal ultrasound waves areprovided which “sonify” the tube in accordance with various angles ofincidence. The angles of incidence are adjusted as a function ofnumerous parameters, including the dimensions of the tube, itsultrasound transmission properties, the types of defect sought, etc.

The recurrence frequency of the bursts is limited by the outward andreturn travel time of the ultrasound waves in the coupling liquid and inthe metal of the tubes. A long travel time therefore makes it necessaryto reduce the recurrence frequency and hence the productivity of thenon-destructive control.

According to some known embodiments, the sensors are fixed in positionand a helical movement is imparted to the tube.

According to other known embodiments, the ultrasound detectors orsensors are driven in rotation at a rate of a few thousand revolutionsper minute about a tube moving at a linear rate which may be as much asapproximately one metre per second.

In yet other known embodiments, a sensor is used which is constituted bya multiplicity of ultrasound transducer elements surrounding the tube.The successive excitation of groups of transducer elements makes itpossible to proceed with the “formation” of an ultrasound beam withwhich an angle of incidence on the tube may be associated. Theexcitation also makes it possible to rotate the beam about the tube byswitching the excited groups of elements, and, as a consequence, toreplace the above-described mechanical rotation of the sensors byelectronic scanning (FR-A-2 796 153).

A particular case of control is that of weldless tubes which areobtained by hot “drilling” bars between cylinders. That manufacturingprocess leads to defects referred to as “oblique” or defects in theshape of a helix, which exhibit some obliquity relative to the axis ofthe tube. The obliquity may be positive or negative, depending on thedirection of the helix.

The obliquity of the defects depends on the manufacturing range which isused and, in some cases, on the stage of formation of the defect. Thus,the same control installation may therefore have to control defectswhose obliquity may vary from −20° to +20°, or more.

The least obliquity brings about major attenuation of the echoesreflected by the defects when the incidence of the beam has beenoptimised to detect strictly longitudinal defects.

The patent U.S. Pat. No. 3,924,453 describes conventional sensors whichcause the ultrasound beam to diverge mechanically in a plane passingthrough the axis of the tube (so-called “toric divergent” process). Therange of obliquity that is detectable is, however, limited.

In addition, the use of multi-element sensors permitting the formationof an ultrasound beam whose deflection is suitable for detecting a givendefect obliquity, theoretically enables that problem to be solved. It istherefore expedient to adjust each burst in such a manner that anoptimum incidence for a given obliquity corresponds to each burst.

Each burst involves a propagation time in water for outward travel, apropagation time (one outward and return journey, or more) in the tubeand, again, a propagation time in the water for return travel. Althoughit would be possible to multiply the bursts as a function of the numberof desired incidences, that technique is in fact hardly applicableindustrially, especially owing to the cumulative propagation times whichthe multiplication of the bursts makes prohibitive. Those cumulativepropagation times are physical characteristics which cannot decreaseover time.

The invention improves the situation by increasing the productivity ofthe control installation while at the same time preserving a high levelof detectability of defects, in particular defects that are obliquerelative to the axis of the tube.

To that end the invention proposes an installation for thenon-destructive control of metallurgical products, in particular withindirect coupling, comprising:

-   -   an ultrasound sensor device comprising a set of selectively        accessible ultrasound transducer elements (Ci),    -   an upstream circuit capable of selectively exciting the        transducer elements at selected instants,    -   a downstream circuit capable of collecting the signals sensed on        return by the transducer elements, and    -   a processing component (which can be incorporated in the        downstream circuit) capable of analysing the signals sensed, as        a global response of a metallurgical product to ultrasound        excitation.

According to one feature of the installation:

-   -   the upstream circuit is arranged to operate by bursts, which are        associated with the same temporal law of excitation of the        transducer elements,    -   the downstream circuit comprises a memory and is arranged to        store samples of the signals sensed (Sij) by each transducer        element, in correspondence with each burst, to a selected        temporal depth, and    -   the processing component is suitable for co-operating with the        memory in order:    -   for each burst, to read and add up repetitively groups of        samples (Sij) corresponding to different transducer elements        (Ci), and also to instants (tj) staggered from one element to        the other, according to a selected temporal processing law,        peculiar to each repetition, which makes it possible to        calculate for each burst a plurality of reconstituted responses        (S_(T), α_(T)), each of which corresponds to a deflection        α_(T)(multi-processing), and    -   to analyse the global response constituted by those        reconstituted responses as a whole.

Thus, a plurality of ultrasound responses each corresponding to a“simulated” incidence, which is selected a posteriori, can be derivedfrom a single burst.

According to one of the advantages afforded by the present invention,the rapidity of the analysis of the metallurgical products is thereforenow limited only to the necessary processing times.

In one advantageous embodiment, the downstream circuit comprises adigitalisation unit for the sensed signals, and the memory is arrangedto co-operate with the digitalisation unit with a view to storing, as afunction of successive instants, on the one hand, and of activetransducer elements, on the other hand, the amplitudes of the signalssensed by each transducer element.

In one embodiment, the downstream circuit comprises calculation meanscapable of defining a distribution of delays to be applied,respectively, to the sensed signals in order to obtain a global responsewhich corresponds to an emission according to a selected beamdeflection.

Advantageously, the calculation means are also arranged to take intoaccount the emission characteristics of the transducer elements, such asthe convergence of the beams emitted by each element, in the definitionof the distribution of the delays to be applied.

The invention is susceptible of different variants, especially thefollowing, which may be combined:

-   -   at each emission burst, the temporal excitation law may not        comprise any phase shift between the transducer elements; it may        also include such phase shifts; it is also possible to provide        for both, that is to say, one burst without a phase shift and        one or several bursts with phase shifts, so long as the        multi-processing of each burst is preserved.    -   the burst(s) with phase shifts may be used to facilitate the        definition a posteriori (after multi-processing) of large beam        deflections for which the attenuation of the ultrasound waves        should be taken into account.    -   a helical relative movement is provided for between the tubes        and the ultrasound sensor device, by displacing the tube or the        sensor or both.    -   the sensor may be one-dimensional, that is to say, a linear bar        of transducer elements which is arranged substantially parallel        with the axis of displacement of the tubes, or a bar which is at        least partially arc-shaped (for example in the shape of a        truncated cylinder or a truncated sector of a cylinder), and        which surrounds the tube.    -   the processing component can be arranged to process the return        signals by distinct groups of elements of the bar.

This constitutes in each case a “virtual sensor”, with the aid of asub-set of the transducer elements of the sensor. Thus, all of theelements of the sensor are fired simultaneously while, at each burst,the signals received by the various virtual sensors are analysed aposteriori, for a (or each) desired phase-shift law.

-   -   the ultrasound sensor device may comprise a two-dimensional        network of transducer elements (not necessarily flat). Columns        and rows may be distinguished therein. The columns and/or the        rows may be used as the above-mentioned one-dimensional sensor.        Such a two-dimensional network of sensors is called a “mosaic”        network.    -   the mosaic sensor can be used to detect oblique defects without        a requirement for a physical helical relative displacement        because it permits “electronic rotation” of the beam.        “Electronic” rotation means a processing of various virtual        sensors which is capable of scanning the circumference of the        products (at least partially, the complement of rotation being,        if necessary, effected by physical displacement). The downstream        circuit is therefore arranged to calculate distributions of        delays both between the elements of the same row and between the        elements of the same column.

Other features and advantages of the invention will emerge on examiningthe following detailed description and the appended drawings in which:

FIG. 1 shows a tube T which has an oblique defect D;

FIG. 2A shows an ultrasound control device with a selected incidence ina longitudinal sectional plane of the tube T, which plane passes throughthe axis thereof;

FIG. 2B shows an ultrasound control device with a selected incidence, ina transverse sectional plane of the tube T;

FIG. 3 shows diagrammatically sensors Ci and the delays τi to be appliedin order to create a deviation α, a priori, of a ray of ultrasound wavesR1;

FIG. 4 represents a graph showing the ultrasound beam incidences to beapplied for an obliquity β of a defect present in a tube;

FIG. 5 shows an ultrasound burst, with a column of water CE embodyingthe interface between the sensor C and the tube T, the ultrasound burstpassing first of all through the water and then through the metal of thetube T;

FIG. 6 shows diagrammatically an installation for detecting defects, inaccordance with the present invention;

FIG. 7 shows diagrammatically the divergence δ of a beam F1 emitted byan ultrasound sensor C, of a given width L;

FIG. 8 shows diagrammatically the selected intensities in the signalsreceived, without taking into account focusing applied to the ultrasoundbeam (vertical hatching) and taking into account that focusing(horizontal hatching);

FIG. 9 is a graph of the global propagation times of the ultrasoundwaves and the calculation times (ordinate in microseconds) as a functionof the number of defect obliquities that can be detected (on theabscissa), in accordance with the technique of the prior art (short andlong broken lines) and according to the invention (continuous line) andby an optimised process, in accordance with the present invention(broken lines);

FIG. 10 shows the amplitude of a signal received (from a non-deviatedinitial beam) for several obliquities β of a defect detected with asensor according to FIG. 6 and a standard sensor used in a conventionaltechnique (curves with dotted line);

FIGS. 10B and 10A are A-SCAN signal curves obtained on oblique defectsby a technique according to the invention and by a technique ofde-multiplied bursts of the prior art, respectively;

FIG. 11 shows the loss of sensitivity (by acoustic attenuation) forlarge obliquities aimed at, with the technique of demultiplied bursts(curve with solid line) and the single-burst technique according to theinvention (curve with dotted line); and

FIG. 12 shows an ultrasound control device in an embodiment using anarc-shaped sensor;

FIG. 13 shows an ultrasound control device in an embodiment using a“mosaic” sensor;

FIG. 14 shows the response of a notch as a function of the relativepositions of the notch and the bar, for virtual sensors that arejuxtaposed and composed of 8 elements; and

FIG. 15 shows the response of a notch as a function of the relativepositions of the notch and the bar, for 50%-imbricated virtual sensorscomposed of 8 elements.

The following drawings and description basically contain elements of adefinitive nature. They can therefore not only be used to make thepresent invention more understandable but they can also contribute toits definition, where appropriate.

Reference will first of all be made to FIG. 1 in which a tube T leavinga production line has an oblique defect D, of obliquity β relative tothe longitudinal axis of the tube. In particular, in a process formanufacturing tubes, without a weld, in which hot-drilling is carriedout on metal bars between cylinders, the tubes T occasionally have suchdefects, in the shape of a helix, with some obliquity β relative to theaxis of the tube T.

Referring to FIG. 2A, a device for the non-destructive control of tubesT comprises a sensor device C which is constituted by one or moreultrasound transducer elements and which “sonifies” the tube T byultrasound emission at a firing rate, called the recurrence frequency,of, for example, approximately 4 KHz. In principle, the coupling betweenthe ultrasound sensor and the metal tube T is indirect. A column ofliquid, in particular water, is generated between the sensor C and thetube, so that the ultrasound waves emitted by the sensor are propagatedfirst of all in the water and then in the tube.

The incidence of the ultrasound beam emitted by the sensors C may bedefined, in space, by two angles of incidence. Referring to FIG. 2A, anincident ray R1 forms with a normal N of the tube an angle α_(T) (axialor longitudinal deflection) in a longitudinal sectional plane of thetube T, which plane comprises the axis of the tube. The second angleenabling a beam incidence to be defined is the angle α_(L) of FIG. 2B.The ray R1 emitted by the sensor C forms an angle α_(L) relative to thenormal N of the tube T. The angle α_(L) (transverse deflection) isdefined in a transverse sectional plane perpendicular to the axis of thetube T.

The defect D is in principle located on the outer or inner surface ofthe tube T or in the vicinity of those surfaces. It comprises alongitudinal component and also a transverse component, the proportionof which is a function of the angle β of the defect. That obliquity β isdefined relative to a generatrix of the cylindrical tube T, parallelwith the axis of the tube, and may be positive or negative.

The tube itself generally has a relative helical movement with respectto the sensor C in order to control substantially the entire surface ofthe tube. Along the axis of the tube, the component of the relativehelical movement is rectilinear, at a speed which is substantiallyconstant and which may be as much as approximately 1 meter/second. Therotary component of the relative helical movement may be generated bythe rotation of the sensor about the axis of the tube or by a rotationof the tube about its axis, the sensor being fixed in position, or elseby a combination of those two rotations. In both cases, the sensor isoriented in such a manner that it fires at the tube in accordance with atransverse deflection α_(L) of approximately 17° in order to detectlongitudinal defects or in accordance with an axial deflection α_(T) ofapproximately 17° in order to detect transverse defects. Thosedeflections α_(L) and α_(T) have different values for a propagationmedium other than water and for tubes made of a material other thansteel. The orientation may be fixed (non-adjustable) or may bemechanically adjustable to some extent, but the adjustment istime-consuming and tricky.

In a recent, more developed, technique, a plurality of transducerelements Ci is used (FIG. 3) whose normal is perpendicular to the tube.Respective emission delays τi are applied to the transducer elements inorder to create a phase shift between the respective emitted elementalwaves Oi, which translates into a difference in travel between therespective emitted waves Ri. The beam that results from all of the wavesemitted therefore has a maximum energy in accordance with a deflectionα, which is managed electronically by controlling the instant at whichthe elements Ci emit. It is thus that the totality of the delays τiapplied defines a distribution of the delays, called the phase law ortemporal law, on the network of sensors Ci and consequently thedeflection α of the emission beam resulting from the various wavesemitted Ri.

The transducer elements are arranged on a bar. Since the pitch pebetween each element Ci is known, it is possible to construct thedistribution of the delays to be applied to the emission on the variouselements (phase law) to obtain a given deflection α, from the formula(1):sin α=V.dt/pein which dt is the delay to be applied between the two consecutiveelements and v corresponds to the speed of a longitudinal ultrasoundwave in water (V=1490 m·s⁻¹). The beam thus formed and deflected by αarrives on the tube in accordance with the incidence α, in other wordsthe angle of incidence on the product is in practice the angle ofdeflection of the beam.

In order to listen to the signal in an optimum manner, that is to say,in the direction of the incidence of emission, the same phase law isapplied to the signals reflected by a defect and received on return bythe various elements Ci.

It is also possible to excite successive groups of elements Ci in orderto carry out electronic scanning, for example around the tube, if theelements Ci are in the shape of an arc of a circle or the like.

In general, oblique defects are difficult to detect at the same time aslongitudinal defects, in particular because an optimised incidence ofthe beam of ultrasound waves for controlling long defects produces agreatly attenuated response on oblique, and even slightly oblique,defects. For example, the attenuation generally exceeds a factor of 2,for a defect obliquity of 5°. Here an attempt is being made to detect,simultaneously, longitudinal and oblique defects (if possible, with anobliquity of from +35° to −35° at least, without a crippling loss ofsensitivity).

The detection of oblique defects therefore requires the adaptation ofthe angles α_(L) and α_(T), which vary with the obliquity of a defect.Referring to FIG. 4, it appears that the optimum for detecting alongitudinal defect (β=0) corresponds to 17° for the angle α_(L), whilethe angle α_(T) is zero. Those values of α_(L) and α_(T) are reversed ofcourse for a defect obliquity of 90° (transverse defect). For example,for an obliquity β of 45°, the angles α_(L) and α_(T) correspond todeflections of approximately 12°, in a transverse plane and in alongitudinal plane (FIG. 2A and FIG. 2B), respectively.

In fact, for obliquities smaller than 30°, the variations in the angleα_(L) are relatively small and may be ignored (variation of 3° of angleat most at the beginning of the decrease of α_(L) as a function of theobliquity β). On the other hand, the introduction of an angle α_(T)permits the detection of oblique defects, with an obliquity smaller than30° in absolute value.

Thus, when an attempt is made to detect oblique defects, the value ofα_(L) will be fixed at 17° and α_(T) will be varied in the longitudinalsectional plane of the tube T, for example within an allowance range offrom −10° to +10°, in order to detect the various possible obliquities,including the obliquity zero (longitudinal defects).

In order to detect oblique defects, the patent U.S. Pat. No. 3,924,453proposes an optical process (called a toric divergent process) whichconsists in causing the beam to diverge, by means of a lens, in thelongitudinal plane of the tube and, on the other hand, in focusing thatbeam in the perpendicular plane (plane of FIG. 2B). Thus, a relativelylimited field of detection is arrived at, of the order of 10° around anobliquity aimed at (−10°<α_(T)<+10°). In addition, that process has thedisadvantage of a sensitivity which is variable in accordance with theobliquity. It is thus possible to detect imperfections which would beacceptable and to allow unacceptable defects to pass.

Another process according to the present invention consists in forming abeam deflected by an angle α_(T) by means of a sensor in the form of abar which comprises a multiplicity of transducer elements, while theangle α_(L) is fixed by the configuration of the cell (preferablyapproximately 17°).

Referring to FIG. 7, a sensor C emits a beam F1 of ultrasound waves, theextreme edge of which forms, with the normal to the sensor C, an angleδ, called the angle of divergence (or of opening). The divergence δ isgiven by the formula (2):sin δ=1.22 λ/L,where λ is the wavelength of the beam in water and L is the width of atransducer element of the sensor (FIG. 7). In any case, the divergenceremains greater than the maximum angle α_(T) (11°) for detectingobliquities of β=30°. That divergence is advantageously used to attainthe desired values α_(T).

For example, for ultrasound waves emitted in water at 5 MHz withelements Ci of width 1.4 mm of a linear bar, δ is approximately 15°.

Using a phase law appropriate to the emission of the elements Ci inaccordance with formula (1) and FIG. 3, it is possible to form a beamdeflected by an angle α_(T), so long as the value of α_(T) is smallerthan the divergence δ given by the formula (2). It is then possible toadjust α_(T) “electronically” by modifying the phase law without itbeing necessary to orient the sensor in that direction. The same phaselaw is then applied to the signals received on return and the signalsreceived on return, which have been subjected in that manner to a phaseshift, are added up to obtain a maximised global response.

That process, envisaged by the Applicant, makes it possible to aim at agiven obliquity, with a correct and known sensitivity, and to have ahomogeneous response for the various possible obliquities of the tubedefects. The table in annex A1 contains the results of preliminary testsin respect of the deflection α_(T) of the beam with a view to detectingoblique defects (multi-burst method).

More precisely, the results relate to the amplitude of the echoesobtained on return for various obliquities of defect and various valuesof deflection α_(T) of the beam and to the amplification gain values ofthe signals. The values indicated in bold correspond to the deflectionvalues to be used to detect a given obliquity. It will be appreciatedthat the results obtained for the amplification gains are satisfactory(23.5 dB for the notch at 25°).

It is also possible to compensate for the variation in detectionsensitivity as a function of the deflection used and therefore toprovide a uniform sensitivity for detecting defects irrespective oftheir obliquity.

On the other hand, for each obliquity aimed at, it is necessary to carryout a grouped firing of the transducer elements of the sensor. Thus, ifseveral defect obliquities are sought, the same number of ultrasoundbursts has to be provided since a delay law specific to emission andreception has to be provided for each obliquity aimed at.

Referring to FIG. 5, an ultrasound burst R1 is composed of travelthrough water T_(e) followed by travel through the tube T_(m) (usefultravel), those travels having durations proportional to the thickness ofthe materials passed through. For acoustic reasons, the column of waterCE which covers the interface between the sensor C and the tube T issuch that the travel time in the water is greater than the travel timein the metal of the tube.

In particular, the total time of an ultrasound burst T_(t) isT _(t) =T _(e) +T _(m), with T _(e) >T _(m).

Applied to the multi-burst detection of oblique defects, if n is thetotal number of obliquities aimed at, the total time T_(t) becomes:T _(t) =n.(T _(e) +T _(m))

For conventional industrial firing rates (close to 4 KHz in order tocarry out a simple control of longitudinal defects), it is possible todetect only approximately one or two obliquities, in addition to thelongitudinal defects by the multiple burst technique in the case ofrelatively thick tubes (approximately 36 mm thick), taking into accountthe propagation times of the ultrasound beams.

The detection of oblique defects in accordance with the presentinvention is based on yet another principle.

According to the invention, the transducer elements Ci of a linear bararranged parallel with the axis of the tube are controlled in such amanner that, on emission, all of the elements are activated at the sametime (substantially without a phase shift). In other words, the“physical deflection on emission” is zero. On the other hand, a“virtual” deflection of the beam is constructed on the return signals bystaggering the instants at which the received signals are added up, foreach element Ci.

Referring to FIG. 6, the transducer elements Ci of the sensor arearranged, in the example described, along a bar 3. The bar is fixed inposition in the control installation and its general direction isparallel with the axis of the tube T to be controlled. The bar isinclined by an angle α_(L) of approximately 17° relative to the normalto the tube in a transverse sectional plane of the tube T (plane of FIG.2B) and a helical movement is imparted to the tube T along its axis. Theangle α_(L) of 17° corresponds to the optimum angle α_(L) for detectingdefects having a small obliquity (β less than approximately 30°).

The elements Ci are excited by a control circuit 1 each to emit anultrasound pulse wave Ri having an ultrasound frequency of the order of5 MHz. Preferably, the elements Ci are controlled in such a manner thatthey emit at the same time, that is to say, substantially without aphase shift. The ultrasound waves reflected by a defect and sensed onreturn by each element Ci are converted into electrical signals Si(t).Those analog signals Si(t) are converted, respectively, byanalog-to-digital converters 2, operating, for example, at 10 times theultrasound frequency, that is to say, 50 MHz.

The analog-to-digital converters 2 are connected to a memory 4 forstoring, to a temporal depth of several tens of microseconds, digitaldata which constitute signal samples Sij (for example in respect ofamplitude) and which are associated, respectively, with instants tj. Inthe notation Sij, the index i corresponds to an identifier of element Ciin the bar 3, while the index j corresponds to an identifier of instanttj.

A calculation module in the installation selects from the memory 4 aplurality of signal samples Sij which are staggered, on the one hand, asa function of the index i of the elements Ci and, on the other hand, asa function of the successive instants tj and adds up the staggeredsignal samples. The addition may be effected, for example, ininstantaneous amplitude or in any other appropriate manner.

The frame carrying the reference 5 in FIG. 6 illustrates, by way ofexample, the manner in which the calculations are carried out and alsoillustrates the structure of the memory 4. The member 5 may be regardedas a processing circuit or component (or as a processing module, withoutthe word module implying any individualisation).

The memory 4 is preferably arranged in addresses associated with acolumn index i (corresponding to the elements Ci) and with a line indexj (corresponding to the successive instants tj).

The processing module 5 calculates the delays dt to be applied betweensuccessive columns i, according to formula (1), by the deflection α_(T):dt=pe·sin α_(T) /V

The delay values dt are of the order of several tens of nanoseconds.

The processing module then selects, with a precision of the order of ananosecond, values of Sij, from a set of columns of elements Ci, whichare staggered temporally. It then adds up those samples at each instanttj in order to define a reconstituted response signal for the deflectionα_(T):S _(tj)(α_(T))=S _(1, j) +S _(2, j+2dt) +S _(3, j+4dt) + . . . +S_(n, j+2(n−1)dt)

Such an addition makes it possible to bring back into phase, at thetransducer elements, signals that were emitted at the same moment andthat have undergone travels with travel times which differ in respect ofdt in the outward journey and in respect of dt on the return journeybetween two elements.

After calculating the value of dt according to formula (1), the additioncarried out above is used to maximise the energy of the beam in thedirection of the deflection α_(T).

The table in annex A2 contains the results of tests of deflection α_(T)of the beam for detecting oblique defects, by the method according tothe present invention. It enables the double deflection values (2α_(T))corresponding to phase shifts of (2dt) on the outward journey and on thereturn journey to be determined. For different obliquities, the value(2α_(T)) to be used corresponds to the amplitude values indicated inbold in the table. The table also provides the gain values for thevarious values of α_(T). Those values are acceptable even for largevalues of α_(T).

In the above, consideration has been given to a constant temporaldifference dt between the successive instants tj, which corresponds to alinear delay law NF (FIG. 8) with dt constant. However, that law doesnot take into account a focusing which may be applied to the beam ofultrasound waves on the tube. In order to take that focusing intoaccount in the delay law, the phase shift dt decreases to a minimum andthen increases to the initial value. Referring to FIG. 8, the selectedintensities Sij form, for a time tj, in the matrix 5 of columns Ci androws tj, a curved line FOC.

The processing means enable a reconstituted response for variousdeflection angles α_(T) to be calculated.

The module 6 of the installation recovers the signals of reconstitutedresponses St(α_(T)) in order to shape a signal which can be useddirectly by a display device 7 (display screen or the like). The device7 then represents a signal called “A-SCAN” comprising pulses ofultrasound echoes, which pulses are reconstituted as a function of timeand for one or more selected deflections α_(T).

According to one of the advantages afforded by the present invention,the total time T_(t) taken to aim at n obliquities, with the notationsused above, is then given by:T _(t) =T _(e) +T _(m) +n.T _(calc),T_(calc) being the calculation time which may be expressed as a functionof T_(m), and henceT _(t) =T _(e) +n.G.T _(m),where G is a coefficient representing the processing rate of the chaincomprising the analog-to-digital converters, the memory 4, the selectionof intensities from the set of stored intensities etc. Thus, the lower Gis, the higher is the processing rate.

According to one of the advantages afforded by the present invention,the travel time in water for (n-1) obliquities is thus suppressed.

With current electronic and computer means, G is always less than 1 andmay be less than 0.5 with ultra-rapid electronic systems. Thus, thelimitations are no longer acoustic but electronic because the limitingfactor becomes, in the present invention, the calculation timenecessitated by the above processing operations. The limitations aretherefore no longer physical but evolutive with the advances in therapidity of electronic circuits.

FIG. 9 represents, for a given tube thickness, the total times Tt fordetecting oblique defects, as a function of the number of obliquitiescontrolled n. The graph is prepared using the technique of the prior artwith demultiplication of the bursts (phase shift between the elementsCi, as of emission, for each deflection α_(T), which corresponds to thecurve with long and short broken lines). The technique according to theinvention is also used with a factor G of 1 (solid-line curve) relativeto a standard electronic system and with a factor G of 0.5 (dotted-linecurve) relative to an ultra-rapid electronic system.

It will thus be appreciated that the smaller factor G is, the shorter isthe time necessary to aim at several obliquities, which enables the tubecontrol rate to be increased, in particular in a chain for controllingtubes after they have been machined.

The sensor device has a length adapted to the control pitch of theinstallation, that is to say, a length of the order of 100 mm in theexample. The defects to be detected have a length which may bedistinctly smaller, for example 20 mm. A defect of length 100 mm, thatis to say, of a length equivalent to the bar (3), creates a signal oneach of the elements of the bar and therefore an intense reconstitutedsignal, by addition. On the other hand, a defect of 20 mm creates asignal on 20% of the elements of the bar and therefore a reconstitutedsignal which is 5 times less intense than that for a defect of 100 mm.

A not incompatible imperfection 100 mm long may therefore be detectedand a defect of 20 mm may be detected slightly or not at all.

In order to compensate for that disadvantage, a solution is used whichconsists in carrying out the processing on only a few elements of thebar, for the same firing of all the elements of the bar. For example,the processing may be carried out on an arrangement of 8 elements of abar of 64 elements, and the processing can be started again on otherarrangements of 8 elements of the bar. The group of 8 elements is calleda “virtual sensor”.

Each arrangement gives correctly, after adding up the signals Sij on the8 elements, a signal of the elemental reconstituted response for thedeflection α_(T). It is possible to retain as the global reconstitutedresponse, the elemental reconstituted signal among the reconstitutedsignals of the various arrangements that has a maximum peak amplitude.Each arrangement of “virtual sensor” elements is derived from theprevious arrangement by translation by a pitch pi, called theimbrication pitch.

Let N_(T) be the total number of elements in the bar and let N_(v) bethe number of transducer elements in the virtual sensor. When theimbrication pitch is from 1 to N_(v), while remaining strictly less thanN_(v), there is an overlapping or imbrication of the arrangements of thevirtual sensor. When the imbrication pitch pi is equal to N_(v), thearrangements of the sensor are disposed side by side. When theimbrication pitch is greater than N_(v) and less than N_(T), the variousarrangements do not totally cover the elements of the sensor.

The processing of the signals on the N_(v) elements of the virtualsensor is carried out a maximum number of times. The number of possiblearrangements is given by the formula (3):N=truncation {(N _(T) −N _(v))/pi}+1For example,

-   -   if N_(T)=64, N_(v)=8 and pi=1, then N=57. In that case, two        successive arrangements have 7 elements in common.    -   if N_(T)=64, N_(v)=8 and pi=8, then N=57. In that case, two        successive arrangements have no common element.    -   if N_(T)=64, N_(v)=8 and pi=4, then N=57. In that case, two        successive arrangements have 4 elements in common, which        corresponds to 50% coverage of the successive arrangements.

After retaining as the global reconstituted response of the Narrangements, the one that has a maximum peak amplitude for thedeflection α_(T) sought, it is in the same way possible to carry out thecalculations for other values of the deflection α_(T).

By way of variation, it is possible first of all to determine thevarious signals of the elemental reconstituted response for variousangles α_(T) and a virtual sensor arrangement and then to carry out thesame calculations for the other arrangements.

In any case, the global reconstituted response for a given deflection isdetermined, retaining the elemental response that provides a maximumpeak amplitude for that deflection.

When the arrangements of virtual sensors that are located at the ends ofthe bar are processed, the response signal is disturbed by the absenceof elements on each side of the virtual sensor. That is why it ispreferable to eliminate (N_(v)/2) elements at each end of the bar, fromthe various virtual sensor arrangements.

The maximum number of arrangements is therefore reduced toN′=truncation {(N _(T)−2N _(v))/pi}+1

As shown by the table of annex A2, the global reconstituted signal isamplified in a different manner for each value of α_(T) so that therigour of the control operation is the same for the various values ofα_(T).

By way of variation, the amplification gain may be uniform, while thetriggering threshold of the defect is adapted to each value α_(T).

The Applicant has carried out “static” tests for controlling obliquedefects, on a steel tube 96 mm in diameter and 12 mm thick. Notches ofapproximately 5% of the thickness of the tube were formed in order tosimulate oblique defects. The obliquity of the notches is from 0° to 25°and their length in the longitudinal direction is approximately 12 mm.

The sensors used are marketed by the company Imasonic under the nameImasonic (registered trademark). The pitch pe between the elements is1.5 mm (with 1.4 mm of width L for one element). The device comprises atotal of 32 elements, with a mechanical focusing of 50 mm in water. Inan example of a cell used, the deflection α_(L) may be fixedmechanically at 17°. The electronic system used is an electronic systemof RDTech make of the type Focus 32/128 which is capable of managing 32elements in parallel.

Bearing in mind the geometry of the transducer elements, theirdivergence δ is of the order of 15° and is entirely suitable for thedeflection values to be used in order to detect oblique defects for anobliquity of from −25° to +25°.

The static tests were carried out using a virtual sensor of 8 elementsfor analysing the return signal. The defect is disposed at right-anglesto the virtual sensor. Such a virtual sensor has a listening length ofapproximately 9 mm, which is very suitable for the length of the defectbeing studied (12 mm).

Referring to FIG. 10, the attenuation of the response of a notch as afunction of its obliquity (0°, 5° or 10°) is smaller than that obtainedwith conventional sensors having a single element approximately 10 mmwide. Typically, for a conventional sensor, the attenuation is at least5 dB for a notch of 5°, while the sensors used here undergo anattenuation of less than 1 dB for the same notch obliquity.

However, for obliquities β greater than +20° or less than −20° (FIG.11), the Applicant noticed a slightly lesser sensitivity in the signalsof the reconstituted response (installation according to the invention)compared with the case where a multi-burst installation would be used,with the same type of bar and with in each case a burst according to thepreferred obliquity sought, which gives an “adapted emission-receptiontravel”.

In fact, in the installation according to the invention, there is nodelivery onto the defect of a beam deflected a priori (on emission) bythe preferred angle α, as may be effected in a multi-burst installation;the invention proceeds merely by phase restoration of the signalsreceived, when they arrive on the transducer elements. Those receivedsignals include the component reflected (strictly, “back-scattered”) bythe defect; and it is the phase restoration which enables the elementalresponse, and then the global response, to be reconstituted for a givendeflection α_(T).

However, that phase restoration on reception relates to signals thathave not been correspondingly brought into phase on emission. It followsthat it relates to received signals which have not been subjectedexactly to the “adapted emission-reception travel”. As a general rule,the tests carried out by the Applicant have shown that, under normaloperating conditions, the relative attenuation (from one sensor elementto the other) which results from that difference in travel wassufficiently small to remain negligible, at the very least in a specificrange of obliquities.

The gain to be used in the installation according to the invention isnot, however, incompatible and, in addition, the gain in controlproductivity for several obliquities largely counterbalances thedisadvantage on the lesser sensitivity.

FIGS. 10A and 10B show an A-SCAN recording of the same defect having anobliquity of 20°, on a multi-burst installation and on a so-called“post-acquisition” processing installation according to the invention,respectively. FIGS. 10A and 10B show that the two types of installationenable the 20° oblique defect to be detected in a similar manner; inthose Figures, the signal EI denotes the water-steel interface echo andthe signal ED denotes the defect echo. The straight-line segment havingan amplitude of 30% corresponds to the defect criterion (temporal gateand intensity).

The Applicant has also carried out dynamic tests in order to determine,in particular, the useful detection regions of the virtual sensors. Thedynamic tests were carried out on the same tubes as above but with anotch 20 mm long having an obliquity β=0° in order to determine thedetection holes. Thus, the obliquity of the defect has no influence onthe measurement. The tests were carried out using the same equipment asfor the static tests and a mechanical installation cell of the type withadjustment of the angle α_(L). The angle α_(L) has been optimised on thesignal stemming from an external longitudinal defect (β=0°). TheApplicant also used a Sofranel 5052GPD gate with a frequency band of4-12 MHz.

FIGS. 14 and 15 show the response of the 20-mm notch as a function ofthe relative positions of the notch and of the bar of 32 elements havinga total length of 48 mm, for two series of arrangements of virtualsensor composed of 8 elements.

For an imbrication pitch pi=8 elements, the immediate successivearrangements do not cover one another and do not overlap one another, asin the conventional configuration with detectors having a diameter of 15mm.

FIG. 14 shows a useful sensor region at −2 dB of 31 mm, that is to say,65% of the bar; the detection holes are lower than 1.5 dB and have alength sufficiently small not to impair the detection of notches 25 mmlong.

For an imbrication pitch pi=4 elements, the immediate successivearrangements cover one another or overlap one another to the extent of50%. No more detection holes are found but the number of arrangementsand the calculation times are also doubled.

The static and dynamic tests carried out show that the installation withpost-acquisition processing according to the invention enables obliquitydefects of from −20° to 20° to be detected.

In order to achieve obliquity values higher than 20°, it is possible touse the installation and the methodology with post-acquisitionprocessing by carrying out a phase shift on the single firing of all ofthe elements of the sensor device in order to form a beam having anon-zero “physical deflection on emission”, for example of 5°.

The processing of the signals sensed on return is similar to theprocessing described above, which comprises storage of the Sij samples,a selection of the Sij values staggered in accordance with a delay lawand an addition of those staggered values for each calculation of areconstituted response under a deflection α_(T). The signal returned bythe defect is then less attenuated for the large deflections α_(T) andit is possible to exploit the capacities of divergence δ of the elementsCi of the bar in the best manner.

However, that method has some disadvantages because the optimisation ofthe deflection α_(L) is no longer effected on a straight defect (β=0°)but on a defect having an obliquity of 20°. In addition, the method doesnot enable the major positive and negative obliquities to be detected,and the displacement of the virtual sensor arrangements may be renderedmore complex by the use of bursts with a phase shift on emission.

FIG. 12 represents another embodiment of an installation according tothe invention which is intended to control longitudinal defects ontubes. According to that Figure, the sensor device is constituted by alinear bar 13 in the shape of an arc of a circle and the variouselements of the sensor are arranged along the arc. The tubes T to becontrolled are moved along their axis and the plane of the bar isperpendicular to the axis of the tubes. Firing is carried out on all ofthe N_(T) elements of the sensor, with or without a phase shift betweenelements. The same post-acquisition processing as that described aboveis carried out in order to determine a reconstituted response for agiven deflection α_(L), and for various virtual sensors in the bar, inorder to effect, for example, an electronic rotation of the beam aboutthe tube, as indicated above.

FIG. 13 represents yet another embodiment according to the invention. Inthat Figure, the sensor device is arranged on a cylindrical surface andis constituted by a mosaic or by a network of elements arranged in rowswhich are parallel with one another, for example 13-1, . . . 13-i, . . .13-n, as illustrated. In the example, each row is a generatrix of thecylindrical surface of the sensor. The tubes to be controlled aredisplaced along their axis coaxially with the cylindrical surface of thesensor.

In FIGS. 12 and 13, the control circuit (or upstream circuit, foremission) is marked 10, and the reception assembly (or downstreamcircuit) is marked 13.

The upstream circuit 10 is arranged to produce the same firing of all ofthe transducer elements, with or without a phase shift between adjacentelements. In the case of FIG. 13, for example, it is possible to carryout a phase shift between successive rows in order to form a beaminclined in an optimum manner in the cross-sectional plane of the tubes(deflection α_(L) of approximately 17°), but without a phase shiftbetween elements of the same row, and to effect an electronic rotationof the beam about the tube using different virtual sensors.

Post-acquisition processing is carried out on the return signals whichhave been stored, according to the invention, in order to reconstitute aresponse signal for optimised angles α_(L) (approximately 17°) andangles α_(T) of from −5° to +5°. Such an installation having a mosaicsensor enables several defect obliquities to be controlled at a fastrate (from −20° to 20°) with a rectilinear advance of the tubes in theinstallation, unlike the previously described installations which use ahelical advance to detect oblique defects.

It is also possible to select in that installation, laws for deflecting,in a variable but selected manner, the return beam in accordance with across-section of the tubes or in accordance with a longitudinal plane ofthe tubes in order to optimise the angles α_(T) and α_(L) (for exampleα_(T)=17° with α_(T)=0°, α_(T)=13° with α_(T)=11°).

It is possible to use two-dimensional virtual sensors in thatinstallation. In that case, the sensors are displaced by an imbricationpitch pi having an axial component and a circumferential component.

The installations described have in common their capacity to increasethe control rate by effecting a single post-acquisition processing ofthe return signals. It is possible to simplify the examples describedand likewise the combinations of the features of the examples described,especially as a function of the types of control which it is desired tocarry out, together or separately.

A1 Amplitude of the echo on the Deflection Gain notch according toobliquity (%) α₁ (°) (dB) 0 5 10 15 20 25 45 0 14.4 90 50 10 1 15.4 7090 40 15 3 15.8 10 55 90 55 20 4 16.8 20 75 90 30 10 5 20 55 >100 90 307 23.5 10 40 83 90

A2 Amplitude of the echo on the Double Deflection Gain notch accordingto obliquity (%) 2 α_(T)(°) (dB) 0 5 10 15 20 25 45 0 11 90 53 10 3 1220 90 20 6 15.5 20 90 10 8 20.5 25 20 35 90 10 11 32.2 80 50 35 50 90 1640 >100 >100 90 65 65 70

1. An installation for non-destructive control of metallurgicalproducts, comprising: an ultrasound sensor device comprising a set ofselectively accessible ultrasound transducer elements; an upstreamcircuit configured to selectively excite the transducer elements atselected instants; a downstream circuit configured to collect signalssensed on return by the transducer elements; and a processing componentconfigured to analyze the signals sensed, as a global response of ametallurgical product to ultrasound excitation; wherein the upstreamcircuit is configured to operate by bursts, which are associated with asame temporal law of excitation of the transducer elements, wherein thedownstream circuit comprises a memory and is configured to store samplesof the signals sensed by each transducer element, in correspondence witheach burst, to a selected temporal depth, wherein the processingcomponent is configured to cooperate with the memory and is furtherconfigured, for each burst, to read and add up repetitively groups ofsamples corresponding to different transducer elements, and also toinstants staggered from one element to another, according to a selectedtemporal processing law, peculiar to each repetition, wherein saidprocessing component is further configured to calculate for each burst aplurality of reconstituted responses, each of which corresponds to adeflection on emission, and wherein said processing component is furtherconfigured to analyze the global response constituted by thereconstituted responses as a whole.
 2. An installation according toclaim 1, wherein the ultrasound transducer elements have a divergence atleast equal to a maximum angle of deflection, for the reconstitutedresponses.
 3. An installation according to claim 2, wherein thedownstream circuit comprises a digitalization unit for the signalssensed by each of the transducer elements of the sensor device.
 4. Aninstallation according to claim 3, wherein the memory is arranged tocooperate with the digitalization unit for storing, as a function ofsuccessive instants, and active transducer elements, the samples of thesignals sensed by each transducer element.
 5. An installation accordingto claim 4, wherein the processing component comprises means fordefining distributions of delays applied, respectively, to the sensedsignals, and for obtaining, for each distribution, a reconstitutedresponse that correspond to a burst according to a selected beamdeflection.
 6. An installation according to claim 4, wherein theprocessing component comprises means for gaining access to the memory asa function of times associated with the samples, for each transducerelement.
 7. An installation according to claim 1, wherein the upstreamcircuit is further configured to excite the transducer elements,substantially without a phase shift between them, in accordance with atemporal excitation law.
 8. An installation according to claim 1,wherein the upstream circuit is further configured to excite thetransducer elements in accordance with a temporal excitation law suchthat said transducer elements have, between them, phase shifts defininga beam deflected on emission, and wherein, based on the temporalexcitation law, the processing component is further configured to definedistributions of delays to be applied to the sensed signals, taking intoaccount the phase shifts between transducer elements on excitation, sothat the reconstituted responses each correspond to a deflectioncentered around an angle of physical deflection of the beam on emission.9. An installation according to claim 1, wherein said processingcomponent is further configured to consider each group of samples addedup as corresponding to a selected sub-set of transducer elements, as avirtual sensor.
 10. An installation according to claim 9, wherein theprocessing component is further configured to calculate plural elementalreconstituted responses of a product to a same burst under a samedeflection, for different sub-sets of the virtual sensor.
 11. Aninstallation according to claim 10, wherein the processing component isfurther configured to calculate the reconstituted response in a form ofa function of the elemental reconstituted responses for the samedeflection and for different sub-sets of the virtual sensor.
 12. Aninstallation according to claim 11, wherein the reconstituted responseof the product to the burst under a deflection is the elemental responsethat has a maximum peak amplitude.
 13. An installation according toclaim 9, wherein the processing component is configured to calculatereconstituted responses for different deflections, with differentsub-sets of the sensor device.
 14. An installation according to claim 9,wherein the sub-sets of the sensor device, for calculating areconstituted response of the product to the burst under a deflection,comprise substantially a same number (N_(v)) of transducer elements. 15.An installation according to claim 14, wherein the sub-sets of thesensor device are selected from the sensor device while excluding ateach end a guard band, of which a number of transducer elements isapproximately half a number of transducer elements of a sub-set.
 16. Aninstallation according to claim 14, wherein two consecutive sub-sets arederived from one another by a translation by an imbrication pitch. 17.An installation according to claim 16, wherein two consecutive sub-setscomprise common elements.
 18. An installation according to claim 16,wherein said processing component is further configured to consider theselected number of sub-sets of the sensor device for calculating areconstituted response under a deflection as corresponding tosubstantially to a maximum number of possible sub-sets for the selectedimbrication pitch and for a number N_(v) of elements selected from(N_(T)-N_(V)) elements.
 19. An installation according to claim 1, forcontrol of metallurgical products of steel tube type, furthercomprising: means for actuating the steel tubes in accordance with ahelical movement about an axis of the steel tubes, wherein theultrasound sensor device is constructed in a form of a linear bar oftransducer elements disposed substantially parallel with the axis of thesteel tubes and arranged such that the ultrasound beam on emission has aselected deflection in a cross-sectional plane of the steel tubes. 20.An installation according to claim 1, for control of metallurgicalproducts of steel tube type, further comprising: means for actuating thesteel tubes in accordance with a rectilinear movement along their axis,wherein the ultrasound sensor device is constructed in a form of alinear bar of transducer elements disposed substantially parallel withthe axis of the steel tubes and arranged such that the ultrasound beamon emission has a selected deflection in a cross-sectional plane of thesteel tubes, the bar being set in rotation about the steel tubes.
 21. Aninstallation according to claim 1, for control of metallurgical productsof steel tube type, further comprising: means for actuating the steeltubes in accordance with a rectilinear movement along their axis, andwherein the ultrasound sensor device is constructed in a form of a barof transducer elements, substantially in a shape of an arc of a circle,arranged around the steel tubes.
 22. An installation according to claim1, further comprising: means for actuating tubes in accordance with arectilinear movement along their axis, wherein the ultrasound sensordevice comprises a network of transducer elements arranged substantiallyin accordance with a cylindrical surface coaxial with the tubes, inplural rows of elements, which rows are parallel with one another andwith the axis of the tubes, and wherein the downstream circuit and theprocessing component are configured to determine distributions of delayson the signals sensed on return by the transducer elements of a sub-setor of the whole of the network.
 23. An installation according to claim1, wherein the processing component is incorporated in the downstreamcircuit.
 24. An installation for non-destructive control ofmetallurgical products, comprising: an ultrasound sensor devicecomprising a set of selectively accessible ultrasound transducerelements; an upstream circuit that selectively excites the transducerelements at selected instants; a downstream circuit that collectssignals sensed on return by the transducer elements; and a processingcomponent that analyzes the signals sensed, as a global response of ametallurgical product to ultrasound excitation; wherein the upstreamcircuit operates by bursts, which are associated with a same temporallaw of excitation of the transducer elements, wherein the downstreamcircuit comprises a memory that stores samples of the signals sensed byeach transducer element, in correspondence with each burst, to aselected temporal depth, wherein the processing component cooperateswith the memory, for each burst, reads and adds up repetitively groupsof samples corresponding to different transducer elements, and also toinstants staggered from one element to another, according to a selectedtemporal processing law, peculiar to each repetition, wherein saidprocessing component further calculates for each burst a plurality ofreconstituted responses, each of which corresponds to a deflection onemission, and wherein said processing component further analyzes theglobal response constituted by the reconstituted responses as a whole.25. An installation for non-destructive control of metallurgicalproducts, comprising: an ultrasound sensor device comprising a set ofselectively accessible ultrasound transducer elements; an upstream meansfor selectively exciting the transducer elements at selected instants; adownstream means for collecting signals sensed on return by thetransducer elements; and a processing means for analyzing the signalssensed, as a global response of a metallurgical product to ultrasoundexcitation; wherein the upstream means comprises means for operating bybursts, which are associated with a same temporal law of excitation ofthe transducer elements, wherein the downstream means comprises a memoryfor storing samples of the signals sensed by each transducer element, incorrespondence with each burst, to a selected temporal depth, whereinthe processing means comprises means for cooperating with the memory,and for each burst, means for reading and adding up repetitively groupsof samples corresponding to different transducer elements, and also toinstants staggered from one element to another, according to a selectedtemporal processing law, peculiar to each repetition, wherein saidprocessing means further comprises means for calculating for each bursta plurality of reconstituted responses, each of which corresponds to adeflection on emission, and wherein said processing means furthercomprises means for analyzing the global response constituted by thereconstituted responses as a whole.